Plugins in directories and sub-directories of the application directory

It should be possible to have directories and sub-directories in plugin-packages (e.g. for plugins containing extensive data-collections). The plugin-manager must be able to handle directories in the plugin-packages (zip-files) when installing and deleting plugins.

Support for plugins in sub-directories of the main application directory has been implemented but is untested. In the release candidate of HeuristicLab 3.3 all plugins are installed into the main application directory.